The present invention relates to a method of determining the color conversion characteristic of an image display device, such as a monitor, which displays color images, and which has a color conversion means converting, on a pixel-by-pixel basis, image data, referring to the color conversion characteristic. The invention also relates to an image display device having a color conversion means using the color conversion characteristic determined by such a color conversion characteristic determining method.
A conventional method of determining color conversion characteristic in an image display device is first described with reference to FIG. 10. FIG. 10 shows the configuration of a device using an image adjustment method disclosed in Japanese Patent Kokai Publication No. H5-48885. In FIG. 10, reference numeral 104 denotes a keyboard, 105 denotes a mouse, 106 denotes an input means, 107 denotes a controller, 108 denotes an input circuit, 109 denotes a memory, 110 denotes a CPU, 111 denotes an output circuit, 112 denotes an image display unit, 113 denotes an original image, 114a to 114h denote processed images, 115 denotes set parameters, and 116 denotes a hardcopy device. The keyboard 104 and the mouse 105 are examples of input means 106. The controller 107 comprises the input circuit 108 connected to the input means 106, the memory 109, the CPU 110, and the output circuit 111. The image display unit 112 is driven by the output circuit 111.
The illustrated color conversion characteristic determining method simulates the operation of the hardcopy device, and displays the image output from the hardcopy device on the image display device, and permits determination of the optimum color conversion characteristic for the hardcopy device, while observing the simulated images displayed on the image display device. The conventional method relates to a hardcopy device, but the concept of the color conversion characteristic determining method can be applied to image display devices. The operation of the color conversion characteristic determining method of FIG. 10 will next be described.
The memory 109 stores a color conversion simulation program. The CPU 110 executes the program stored in the memory 109. First, image data used for the color conversion simulation is input. The input image data is displayed on the screen of the image display unit 112. Then, the specific manner of processing is input by the use of the input means 106, and the color conversion simulated by the hardcopy device 116 is performed on the input image data, according to the designated manner of processing. The color-converted, processed images 114a to 114h are displayed on the screen of the image display unit 112, together with the original image 113. When the parameters for the color conversion are changed in multiple steps, the processed images are displayed together, arranged in an array. By selecting the processed image which is closest to the original image 113, the optimum parameters, i.e., the optimum conversion characteristic can be determined. When a plurality of color conversion parameters are determined, the operation similar to that described above is repeated for each of the parameters to determine the value of each of the parameters, in turn. The-thus determined color conversion parameters are sent to the hardcopy device.
The above color conversion characteristic determining method simulates the color conversion according to the specific manner of processing, designated by the input means 106, and permits the selection of the optimum image among the plurality of processed images, to thereby determine the color conversion parameters, i.e., the color conversion characteristic, so that it has an advantage that the operator can determine the color conversion parameters, i.e., the color conversion characteristic easily.
However, because the color conversion characteristic is determined according to the visual perception of the operator, it is not suitable for the determination of the color conversion characteristic for bringing the color reproduction closer to that of the standard, such as the sRGB standard, or NTSC standard. Moreover, the operator needs to have a clear idea of the image in the target color reproduction. If the target color reproduction is given in terms of numerical values, such as the chromaticity of the three primary colors, or the chromaticity of the white, then the operator cannot select the suitable one from among those obtained by simulation.
If the target color reproduction is given only in terms of numerical values, such as the chromaticity of the three primary colors, or the chromaticity of white, the method shown in FIG. 11 may be used. FIG. 11 shows a color conversion characteristic determining method which can be applied to an image display device including a color conversion characteristic holding means holding the color conversion characteristic, a color conversion means converting, on a pixel-by-pixel basis, the image data referring to the color conversion characteristic, and an image display means displaying the image data obtained by the color conversion at the color conversion means. This method is described next.
First, a color conversion characteristic candidate which is a candidate for the color conversion characteristic ultimately adopted is selected. Then, the color conversion characteristic candidate is set in the color conversion characteristic holding means. Then, tristimulus values according to the XYZ color system are measured for the colors displayed on the image display means, for each of a predetermined number of the image data (data having the same color throughout the screen). On the other hand, tristimulus values in the target color reproduction for the corresponding image data are calculated. The tristimulus values obtained by measurement and the tristimulus values obtained by calculation are compared. If the result of the comparison indicates “NG (the predetermined condition is not satisfied)” a new color conversion characteristic candidate is selected, and set in the color conversion characteristic holding means. If the result of the comparison indicates “OK (the predetermined condition is satisfied)”, then the color conversion characteristic candidate being used is output as the color conversion characteristic, and the processing for determining the color conversion characteristic is terminated.
For the measurement of the tristimulus values of the colors displayed on the image display means, it takes several seconds for each color. In the case of a high-precision measurement, it takes one minute or longer for each color. If the measurement is performed for each of 60 kinds (60 colors) of image data, and the tristimulus values are measured for each color, and it takes 10 seconds for each color, then it takes 10 minutes for the 60 colors. If the selection of the color conversion characteristic candidate through the comparison of the tristimulus values are repeated 50 times, the total time required is 500 minutes. The total time required for the determination is the sum of the time required for the measurement, and the time required for other works. When a high-precision measurement is performed, or when it is necessary to measure the tristimulus values of the colors displayed for a greater number of image data, or if it is necessary to repeat the selection of the color conversion characteristic candidate through the comparison of the tristimulus values a greater number of times, then even a longer time is required.
Thus, in the method in which a color conversion characteristic candidate is set in the color conversion characteristic holding means, and the tristimulus values of displayed colors are measured, it takes a long time for the determination of the color conversion characteristic, including the time for measurement of the tristimulus values of displayed colors.